SUMMARY OF WORK Epidemiological observations revealed a high prevalence of elevated arterial blood pressure during colder months, however the mechanisms of this seasonal hypertension were not addressed. The overall objective of this project is to develop an experimental animal model of cold-induced hypertension and to investigate the mechanisms responsible for elevation of blood pressure during cold acclimation. Previously we established the temporal relationship between cold-induced (6 C) increase of heat production and systolic blood pressure in adult rats. Aged rats did not respond to cold exposure with an increase of heat production and did not increase the systolic blood pressure in cold. The objective of experiments during this year was to test the hypothesis that sustained cold-induced shivering results in an increase of blood pressure. Adult Wistar rats, acclimated to thermoneutrality (26 C) were subsequently acclimated to 16 C, 10 C, and 6 C. Similarly with acute acclimation to 6 C, gradual acclimation to 6 C resulted in weight reduction and in increase of daily water consumption. However, contrary to acute acclimation, gradual acclimation was not accompanied by sustained increase of heat production and did not result in the increase of systolic blood pressure. Gradual acclimation to cold in rats is achieved through activation of nonshivering thermogenesis rather than shivering. Taken together with last year results that showed that aged rats whose ability for shivering was compromised, these results suggest that sustained shivering is essential for developing the hypertension associated with cold-acclimation.